1. Field of the Invention
The present invention relates to large-sized bismuth-zinc-borate nonlinear optical crystals, and more particularly to large-sized and high-quality bismuth-zinc-borate (Bi.sub.2 ZnB.sub.2 O.sub.7) single crystals grown from bismuth-zinc-borate (Bi.sub.2 ZnB.sub.2 O.sub.7) compound melt or from Bi.sub.2 O.sub.3 flux, and to the applications of nonlinear optical devices made of Bi.sub.2 ZnB.sub.2 O.sub.7 crystals.
2. Description of the Prior Art
In laser technology, the laser wavebands obtained directly from emissions by laser crystals are limited. Therefore, there still exist blank, unused wavebands within the ultraviolet to infrared spectral zone. By utilizing nonlinear optical crystals, these limited laser wavelengths can be converted into new laser wavebands through nonlinear optical techniques, which include frequency doubling, sum-frequency, optical parameter oscillation. By these techniques, blank spectral zones of wavelengths emitted by various laser devices can be exploited, and applications of laser generators can be more intensively developed.
An all-solid-state blue-green laser system could be realized by generating a near-infrared wavelength laser beam from a solid-state laser apparatus and subsequently converting the frequency of the near-infrared laser by using a nonlinear optical crystal. Such systems have great potential for applications in the field of laser technology and are of substantial economic value.
Until now, the main nonlinear optical materials used for the frequency conversion within the blue-green waveband have been KTiOPO.sub.4 (KTP) crystal, β-BaB.sub.2 O.sub.4 (BBO) crystal and LiB.sub.3 O.sub.5 (LBO) crystal. These materials have disadvantages with respect to crystal growth. Specifically, KTP and LBO need to be grown by a flux method because they are non-congruent melting compounds; BBO also needs to be grown by the flux method because BaB.sub.2 O.sub.4 has a phase transition and BBO only exists in a low-temperature phase. The flux method refers to a method in which precursor intentionally contains an excess component or a non-product appearing component. These excellent nonlinear optical crystals have low growth rates, are difficult to obtain as large-sized crystals and incur high costs because it is necessary to grow them by the flux method. These disadvantages of the contemporary nonlinear optical materials have unfavorably influenced large-scale development of all-solid-state blue-green laser systems. Therefore, not only the optical properties and mechanical properties of a crystal have received a lot of attention, but also the preparation method of the crystal has received more and more attention in recent years.
As a general rule, a new crystal is expected to be easily prepared. It is better for that crystal to be a congruent melting compound in order to grow the single crystal by a melt method. Thereby, low-cost, large-sized and high-quality nonlinear optical crystals can be obtained.
The existence of the bismuth-zinc-borate (Bi.sub.2 ZnB.sub.2 O.sub.7) compound, the powder X-ray diffraction data and the powder neutron diffraction data of the compound were reported by the Canadian, Jacques Barbier (Chem. Mater., Vol. 17(12), 3130-3136 (2005)). In order to measure the essential physical properties (including the nonlinear optical properties) of a single crystal, it is required that a single crystal has a size as large as several millimeters and, preferably, a size at the centimeter-level. Until now, a successful preparation of Bi.sub.2 ZnB.sub.2 O.sub.7 single crystals in a size large enough for the measurement of physical properties has not been reported. Furthermore, it is impossible to purchase these crystals in the market. To date, neither the measurement results of the nonlinear optical properties of Bi.sub.2 ZnB.sub.2 O.sub.7 single crystals nor the application of Bi.sub.2 ZnB.sub.2 O.sub.7 single crystals to the fabrication of nonlinear optical devices has been reported.
The present application refers to and hereby incorporates by reference the following references:                Jacques Barbier, et al, “Melilite-Type Borates Bi.sub.2 ZnB.sub.2 O.sub.7 and CaBiGaB.sub.2 O.sub.7” Chemistry of Materials, 17, pp. 3130-3136, 2005.        Ming Li, et al, “Synthesis, Crystal Structure and Optical Properties of Non-centrosymmetric Borate, Bi.sub.2 ZnB.sub.2 O.sub.7” Journal of Synthetic Crystals, 36, pp. 1005-1010, 2007.        Ali Hussain, et al, “Specific features of second order optical susceptibilities for a complex borate crystal Bi.sub.2 ZnB.sub.2 O.sub.7: Experiment and theory”, Current Opinion in Solid-State and Materials Science, 11, pp. 33-39, 2007.        
In the discussion of the present invention, space group of a crystal refers to a mathematical description of the symmetry inherent in the structure.
The growth of this crystal may be carried out using contemporary techniques such as Bridgeman growth or Kyropoulos growth or Czochralski growth or growth using the gradient freeze method.
In the Czochralski growth method, a compound is placed in a crucible, and the crucible is heater by a heating element so that the compound is melted. Then, a seed crystal is lowered gradually to touch the liquid surface slightly. At this time, the seed crystal is rotated slowly and drawn upwardly to draw out the grown crystal.
In the Kyropoulos growth method, the seed crystal is not rotated during the crystal growth and is not drawn upwardly. On the contrary, the seed crystal is solidified, cooled and shrunk in the crucible.